Has Rowling ever recognised mistakes or inconsistencies?

Question

This may be subjective on my part (hence the question) but every time I read a comment or interview by Rowling I get the impression she’s building on the idea of “I had it all planned out from the beginning” (warning, TV Tropes link) and back justifies a lot of stuff in very weird ways.

Other than acknowledging being bad at math, has she ever acknowledged openly something on the lines of “yeah, when I wrote that part this complete other side of the plot hadn’t been fully defined so I had to adjust a bit to make it work”? Or, “yeah that part about Slytherin being mostly bad became a pain later on when the series evolved.”

Note: this is not meant as a rant against her. 99% of the quotes I see for Rowling are on this site. As such it makes sense my perception is all are justifications as they are brought up to precisely answer a question on this site. I’m curious if she’s normally open about her work or not.

Answer 1

Yes.

• As explained in “Why did Harry's father emerge before his mother in HP and the Goblet of Fire?”, early editions of Harry Potter and the Goblet of Fire have a mistake in chapter 34 where the echoes of Harry's mother and father appear in the wrong order. The mistake has been fixed in later editions, and Rowling acknowledged that this was an error that she made together with her editor.

• She also may have admitted that she made a mistake about Dudley's Playstation.

Answer 2

Rowling typically stuck to having it planned out, at least the direction on where the ending would be. She admits to having some continuity errors now and then (as we know, the "bad at maths" is among the most popular of those admissions).

As for plot lines one major example was changes she made to Chamber of Secrets which were moved to Half-Blood Prince:

Certain crucial pieces of information in book six were originally planned for 'Chamber of Secrets', but very early on (first draft of Chamber) I realised that this information's proper home was book six. I have said before now that 'Chamber' holds some very important clues to the ultimate end of the series. Not as many as six, obviously, but there is a link.

Internet Archive Source

She also admits to "Ghost Plots" to which some details may have leaked into the published works but the idea was never fully fleshed out or dropped:

This [Ghost Plots] is a personal expression, which has nothing to do with tales of the dead.

Over the seventeen years that I planned and wrote the seven Harry Potter books (not to mention Quidditch through the Ages, Fantastic Beasts and Where to Find Them and The Tales of Beedle the Bard), I generated a mass of information about the magical world that never appeared in the books. I liked knowing these things (which was fortunate, given that I couldn’t stop my imagination spewing it all out) and often, when I needed a throwaway detail, I had it ready because of the background I had developed.

I also found myself developing storylines for secondary (or even tertiary) characters that were superfluous to requirements. More of a wrench were the plots I worked out for some much more important characters that had to be sacrificed for the bigger story. All of these I inwardly termed ‘ghost plots’, my private expression for all the untold stories that sometimes seemed quite as real to me as the ‘final cut’. I have occasionally been in conversation with a reader and made mention of part of a ghost plot; looks of consternation cross their faces as, for a split second, they ask themselves whether they have accidentally skipped twenty pages somewhere. I apologise to anyone I might have accidentally wrong-footed in this way; the problem is, literally, all in my head.

Wizarding World - Writing by J.K. Rowling - Ghost Plots